Desferri-ferrichrome

Figure 6.18. MacrcKyclic complex fonners. (a) Structure of a ferrichrome (desferri-ferrichrome), one of the strongest complex formers presently known for Fe(III). The iron-binding center is an octahedral arrangement of six oxygen donor atoms of trihy-droxamate. Such naturally occurring ferrichromes play an important role in the biosynthetic pathways involving iron. Complexing functionalities of some biogenic ligands (b) hydroxamate siderophores, (c) catechol siderophores, and (d) phytochelatines. For detailed structures see Neilands (1981).

The formation of A-cis crystals did not necessarily exclude the existence of the other diastereomer in an equilibrium. Attempts to resolve isomers of Cr-desferri-ferrichrome and Cr-desferri-ferrichrys in only resulted in one fraction, which has exactly the same CD spectrum as the model complex A-cis Cr(men)3, with similar As values of the Ea transition bands (Fig. 27). This indicated, together with crystallographic data, that ferrichrome in solution has an overall A-cis configuration 145). 

Figure 1 Representative siderophores of the hydroxamate and catecholate classes. The hydroxamates are synthesized from the amino acid ornithine that has been modified through hydroxylation and acetylation. Ferrichrome (a) is a prototypical example of the tri-hydroxamate class. Structurally, ferrichrome is a cyclic hexapeptide that consists of three modified ornithine residues (each of which has a hydroxamate side chain) and three glycines. Ferrichrome coordinates ferric iron through its three bidentate hydroxamate side chains. Triacetylfusarinine C (b) is also a cyclic tri-hydroxamate, but the three modified ornithine residues are joined by ester linkages rather than by peptide linkages. Ferrioxamine B (c) is a linear tri-hydroxamate consisting of three peptide-huked modified ornithine residues. Enterobactin (d) is a prototypical example of a catecholate siderophore. It consists of a tri-ester ring from which extend three side chains of chhydroxybenzoyl serine. Each of these siderophores binds ferric iron in a hexadentate manner, which results in full saturation of d orbitals and a very stable complex. Ferric forms are shown in (a) and (b). Desferri-forms are shown in (c) and (d)...

---